1. Field of the Invention
This invention relates to a method for partially smoothing retouch techniques in electronic reproduction of color images, and more particularly to the production of corrected and retouched color separations by means of an electronic image processing system.
2. Prior Art
In electronic reproduction, three signals representing measured primary color values are produced by a color scanner by means of opto-electronic scanning, such signals representing the color components red, green and blue of the scanned image points. A color correction computer corrects the color by altering the measured value signals and generates color separation signals therefrom which are required for the production of color separation. Color separation signals represent a measure of the amounts of ink required in the later printing. The color separation signals are digitized and deposited in a storage medium as digital color values of each point in an image.
In an image processing system, the stored color values of various individual masters can be united according to a layout plan to form a data set for an overall page. Also, partial retouches (in which the color value and/or tint value is corrected) can be carried out. Partial retouches, i.e., retouches limited to selectable, limited image parts, are necessary in order to optimize the correction which has been carried out in the color correction computer or to make editorial changes.
For the purpose of recording the retouched color separations, the corrected color values are read out of the memory medium, then converted back into analog color separation signals, and then supplied to a color scanner in which the rastered or unrastered color separations "magenta", "cyan", "yellow" and "black" are exposed for the production of printing forms.
A method for electronic partial retouch has been proposed in the U.S. Pat. No. 4,393,399 in which the digital color values are changed under visual control on a color monitor, image-point-by-image-point. The disclosure of this patent is incorporated hereinto by reference. Such retouching is partially in accord with the desired retouch effects in the color image, or in the color separation, by correction values which are image point-dependent. The image point coordinates of the color values to be retouched, and the desired amount of retouch, are determined with the assistance of a coordinate pen of a coordinate identification device. To this end, the retoucher guides the coordinate pen across the image part to be retouched like a retouch brush, and the retouch degree for the individual image points is derived from the positions where the coordinate pen contacts the corresponding image points. In order to be able to retouch larger image parts more quickly, the plurality of image points covered with each contact of the coordinate pen within an image point area must be increased, this corresponding to an enlargement of the "brush surface".
When copying images with sharp transitions from one surface into another, the subjective impression of a relief image arises, suggesting the impression that the images were not copied into one another but rather, onto one another. Therefore, a typical retouch job requires, among other things, the partial defocusing, flattening or weakening of such contours (density skips).
Another retouch job is the partial matching, leveling or smoothing of different color or tint values of an image.
Although flattenings and matchings can be carried out with the retouch method specified in the cited patent, such a process is time-intensive, particularly when smooth transitions without noticeable density stages are to be achieved, since the coordinate pen must be conducted over the image part to be retouched in movements which lie close to one another.